1. Field of the Invention
This invention relates to the field of disk drive suspensions. More particularly, this invention relates to the field of microactuator grounding in disk drive suspensions.
2. Description of Related Art
Magnetic hard disk drives and other types of spinning media drives such as optical disk drives are well known. FIG. 1 is an oblique view of an exemplary prior art hard disk drive and suspension. The prior art disk drive unit 100 includes a spinning magnetic disk 101 containing a pattern of magnetic ones and zeroes on it that constitutes the data stored on the disk drive. The magnetic disk is driven by a drive motor (not shown). Disk drive unit 100 further includes a disk drive suspension 105 to which a magnetic head slider (not shown) is mounted proximate a distal end of load beam 107. Suspension 105 is coupled to an actuator arm 103, which in turn is coupled to a voice coil motor (VCM) 112 that moves the suspension 105 arcuately in order to position the head slider over the correct data track on data disk 101. The head slider is carried on a gimbal which allows the slider to pitch and roll so that it follows the proper data track on the disk, allowing for such variations as vibrations of the disk, inertial events such as bumping, and irregularities in the disk's surface.
Both single stage actuated disk drive suspensions and dual stage actuated (DSA) suspension are known. In a single stage actuated suspension, only voice coil motor 112 moves suspension 105. In DSA suspensions such as the DSA suspension 105 of FIG. 1, in addition to voice coil motor 112 which moves the entire suspension, at least one microactuator 114 is located on the suspension in order to effect fine movements of the magnetic head slider to keep it properly aligned over the data track on the spinning disk. The secondary microactuator(s) provide much finer control and much higher bandwidth of the servo control loop than does the voice coil motor alone, which is only capable of effecting relatively coarse movements of the suspension and hence the magnetic head slider. The microactuators are commonly linear piezoelectric devices, particularly lead zirconate titanate (PZT) devices. The microactuators will henceforth be referred to herein as PZTs for simplicity of discussion, it being understood that the microactuators can be other types of devices as well.
Recent designs locate the PZTs on the suspension gimbal which holds the read/write head, or to a position in which the PZTs act directly on the gimbal. Such suspensions are called gimbal-based DSA, micro DSA or GSA suspensions. GSA suspensions do not have as great an arm length through which the PZTs act, and hence do not exhibit as much movement of the head slider in response to a unit of voltage input to the PZT microactuators, as do more traditional DSA suspension designs in which the PZTs are mounted at the mount plate such as in the suspension of FIG. 1. However, as data tracks have become narrower and narrower, hence reducing the need for as great a movement of the head slider, GSA designs have become a more attractive option to suspension designers.
Without admitting that FIG. 2 is “prior art” within the legal meaning of that term, FIG. 2 is a bottom plan view of the distal portion of a GSA suspension flexure 20 according to a previous design by the present applicant. As used herein, the term “bottom view” or “bottom plan view” means viewing the suspension from the side on which the slider is mounted. The flexure 20 is usually welded near the distal end of a load beam such as load beam 107 in FIG. 1. With the flexure 20 of FIG. 2 which includes two PZT microactuators 14, those PZTs would eliminate the need for the two PZTs 114 show in FIG. 1.
Two PZTs 14 are affixed to a rigid and non-gimbaled portion of the suspension at their proximal ends. As used herein, the term “proximal” means closest to the actuator arm at which the suspension is mounted, i.e., toward the left in FIG. 2, and “distal” means closest to the cantilevered end of the suspension, i.e., toward the right in FIG. 2. PZTs 14 act directly on the gimbaled portion of the suspension which includes PZT connecting arms or connectors 30 which are attached to the distal ends of the PZTs 14, and slider tongue 52 which carries head slider 54. Flexible connectors 30 act as connectors to transmit tensile and compressive forces, and thereby transmit push/pull movement of PZTs 14 to the gimbaled portion or the flexure specifically to the slider tongue 52 to which head slider 54 is mounted, the connectors 30 being flexible enough to allow the gimbal to pitch and roll relatively freely and thus not interfere with the normal gimbal action as head 54 slider pitches and rolls in response to surface irregularities in the surface of the data disk. Such a suspension is generally disclosed in U.S. Pat. No. 8,879,210 which is assigned to the assignee of the present application. Details of the electrical and mechanical connections to the PZTs 14 are omitted from the drawings of the present application for simplicity of illustration.
The term “stroke” or “stroke length” or “stroke sensitivity” refers to the effective amount of expansion that a PZT exhibits in response to an applied input voltage. There are advantages to high stroke length. One advantage is that a greater stroke length means that the read/write head can be servo actuated through a greater distance over the data platter, and hence can read a greater amount of data from the disk, using only the PZT microactuators which have a high servo bandwidth without needing to actuate the VCM which has a comparatively low bandwidth. Greater stoke length therefore translates into greater seek and read speeds.
If the power and ground electrical connections to the PZT microactuator are less than ideal, actuation voltage potential is lost across those connections and hence stroke length suffers. It is therefore a design goal to minimize electrical resistance of the PZTs' power and ground connections.
Without admitting that FIGS. 3 and 4 are “prior art” within the legal meaning of that term, those figures show two different grounding schemes that have been proposed by the assignee of the present application. FIG. 3 is a cross sectional view taken along section line 3-3′ in FIG. 2. The PZT bottom electrode is electrically connected to a driving voltage by conductive epoxy 32 which extends to copper signal pad 34 having gold plating thereon. As used herein, the term “bottom electrode” will refer to the electrode of the PZT that is farthest away from the flexure and closest to the disk platter, and the term “top electrode” will refer to the electrode that is closest to the flexure. The copper signal pad 34 is part of the flexure's flexible circuit. The ground connection is made through an area of the flexure which has no polyimide and no copper on it. The PZT top electrode is connected to ground through conductive epoxy 42 directly to the grounded stainless steel substrate 46, optionally with a layer 47 of gold plating on the stainless steel for corrosion resistance. Stainless steel substrate 46 is sometimes referred to as the stainless steel support layer of the flexure. The flexure is affixed to load beam 12.
FIG. 4 is a cross sectional view showing an alternative flexure grounding connection, also according to a prior design by the present applicant. Both copper signal pads 34, 36 are part of the flexure's flexible circuit, with copper pad 34 being connected to a driving voltage and copper pad 36 being connected to ground. In this design the ground connection to PZT 14 is made using conductive epoxy 42 adhered to copper contact pad 36 of the flexure over polyimide layer 38. Gold layer 37 is usually plated onto the copper contact pad 36 for corrosion resistance. The copper contact pad 36 is grounded at a ground connection contact point that is not shown through any of a variety of well known structures and methods for connecting copper contact pads 36 within a suspension flexure to ground.
Neither of those two prior designs is ideal. The design of FIG. 3 requires plating gold onto the stainless steel support layer, which requires a number of additional process steps such as dry film resist (DFR) lamination and development prior to plating, and subsequent DFR removal. Such additional steps increase the cost of the flexure.
The design of FIG. 4 can utilize the existing processes of the flexure manufacturing without requiring extra process steps for gold plating, and thus that design enjoys lower manufacturing cost. However, due to the extra thickness of the copper pad 36 and the polyimide layer 38 under PZT 14, and the elastic modulus of polyimide layer 38, the PZT stroke can be significantly compromised.